Why the islands move.

Micronesian navigators routinely make voyages across large expanses of open ocean. To do this, a navigator must judge both the direction in which he is sailing and the distance he has travelled. The rising and setting points of the stars (and other cues) provide instantaneous information about direction, but distance can only be judged by integrating velocity-related information over time. Micronesian navigators judge distance in a way that seems odd. When they are out of sight of land, they imagine that the canoe is stationary and that the islands move back past them. For each voyage, they 'attend' to an island off to the side of the course which is out of sight over the horizon. As they sail, they imagine the island moving back along the horizon changing in bearing until it is imagined to be under the bearing it is known to have from the destination island. Then they know they are near their destination. There is good reason for using a frame of reference whose origin is defined by the boat. We show how it finesses a perceptual paradox--the rising and setting points of the stars do not exhibit motion parallax.